I know the pain of losing a therapist you get on well with. I guess the best thing is to kindly and softly acknowledge your pain around it and know it is normal and be the best kind friend you can to yourself as you go through it. Of course, how you feel about it, depends on what losses you have had in your life and how healed they are, - if you get that.
I send hugs. It will be difficult but even I know that usually when you lose a therapist, you find something better. Strange but true. so you could perhaps keep that in mind, that something better is coming into your life to fill the gap he leaves. HUGS